Probing a Network

ABSTRACT

A method is described of probing a network route using a pairs of probe packets in which the first packets are typically smaller than the second. A first pair of probe packets with size ratio [L 1 /L 2 ] and a second pair of probe packets with different size ratio [M 1 /M 2 ] are transmitted onto a network route. An indicator of time taken for the second probe packet in the packet pairs to traverse the network route is derived and it is identified whether the values of the derived indicators vary or are substantially the same. The method, which can be used to probe home or domestic networks, indicates whether the first link in the route is or is not a bottleneck link, thereby providing further information about the route. A test apparatus, gateway device and computer to perform the method are also disclosed.

The invention relates to a method of probing a network route using apair of probe packets, to a gateway device for a first network and alsoto a test apparatus configured to probe a network route in a remotenetwork, a device configured to probe a network route in a network and acomputer program product.

BACKGROUND TO THE INVENTION

Network probing for capacity is commonly performed using one of manyavailable methods, for example the Variable Probing Size (VPS) methodand the Probe Gap Model (PGM) method.

VPS probing measures round-trip times (RTT) for variable packet sizes,and deduces capacity from the different RTTs measured. VPS uses the IPTime-To-Live parameter to measure individual hops. One major downside ofVPS is that it does not work correctly when the path of the probeincludes multiple network links, or multiple switches. VPS can probelink by link but only on layer-3 routes, as is known in the art, andtypically because of this VPS only probes between routers in an IPnetwork, while aggregating capacity on other, non-layer-3 links. Oneresult of this is that VPS can underestimate path capacity. The wayaround this is to break down the network and apply VPS probing to theindividual links that make up a path. This increases considerably thepre-knowledge needed to apply the method effectively to a network andthe complexity of the method applied in an individual case. However,even this breaks down if switches or other layer-2 or layer-1 devicesare included in the network. A domestic network typically includes suchnon-layer-3 devices.

An alternative, PGM probing, uses 2 back-to-back probe packets of thesame size. Sending two packets back-to-back means that there is notransmission delay between the two sent packets. This is known by theskilled person. However as these packets travel across various networklinks to their destination they will suffer various delays.

Serialization delay in a network is the delay caused by the bandwidth ofthe medium on which packets are sent. It is the time that is needed totransmit the packet. For a packet of size L (bits) at a link oftransmission rate C (bits/s), the serialization delay is equal to L/C.For example, to send a packet of 10,000 bits on a link with atransmission rate of 10,000,000 bits/second, it will take 0.001 secondor 1 millisecond to transmit the packet. Serialization delay isdependent on the packet size and is the time it takes to actuallytransmit a packet.

Queuing delay is the delay suffered by a probing packet because of crosstraffic. If multiple data streams are sent across the same network link,they are normally queued and buffered and then transmitted one packet ata time on the network link. This can mean that a probing packet isbuffered for a certain amount of time, awaiting its turn to betransmitted. This delay is called queuing delay.

Note that such queuing delay can occur in any device on the path betweensender and receiver, including the sender itself. Since this delay isdependent on other data packets, it is independent on the probing packetsize.

Propagation delay is the time it takes for a packet to physicallytraverse a network link and is dependent on the medium used fortransmission but independent of the packet size. For example, on a 50meter Ethernet cable (CAT-cable), the propagation delay is 50m/177,000,000 m/s=0.28 μs. Putting a 1500 byte packet (maximum normalEthernet packet size) on a 1 Gbit/s network link causes a serializationdelay of 1500*8 bits/1,000,000,000 bits/s=12 μs. The propagation delayin this example is only 2.3% of the serialization delay. Thus, unlesspackets travel for very long distances or on very high network speeds,the propagation delay is negligible in most cases when performingnetwork measurements.

There is also a processing delay to be taken into account. To enablemeasurements the probe sender and the probe receiver put a timestamp ona probe packet for ‘packet sent’ and for ‘packet received’. However, thetimestamping of the packet usually occurs due to software in the deviceand is not part of the network interface card itself, thus, when sendinga packet, there is a small amount of time between the time of puttingthe timestamp on the packet and the actual transmission of the packet onthe network. Similarly, there is a small amount of time between thereceiving of a packet and the time of putting the timestamp on it andthis small difference is called processing delay.

A final delay to be taken into account is probe reply delay, the delaycaused by a receiver of a probe packet. It is the time taken to receivea probe and sent out a reply.

For determining a path's capacity with PGM, the queuing delay must bezero. Assuming the cross traffic having stochastic properties, a longenough series of probing measurements will yield at least onemeasurement where the queuing delay is zero. The measurement isidentified by taking the minimum delay of probe packets of a number ofmeasurements. If not negligible, the propagation delays of the PGMprobes will be equal since both probe packets travel the same links.Furthermore, since the packets are of equal size, processing delays,probe reply delays and serialization delays will also be the same forboth packets.

What is different, however, is the serialization delay for each networklink. Network links with higher speeds have smaller serialization delayswhile network links with lower speeds have higher serialization delays.As both probe packets travel across different links, they are dispersedbased on the serialization delays. The initial dispersion is caused bythe first link, as the probe sender transmits the packets on thenetwork. Each time the probe packets encounter a network link that isfaster than at least one previously travelled slower link, thedispersion remains the same. But, each time the probe packets encountera network link that is slower than any previously travelled link, thedispersion will increase due to the longer serialization delay.

Since the size of the dispersion between the two probe packets at thereceiver is determined by the slowest network links, this means that PGMcan be used to measure the bottleneck link on the probe path and in factPGM can only measure the bottleneck link.

Capacity on this bottleneck link can be determined using well knownformulae, as is known in the art.

The most advanced variety of PGM is described, for example, inDelphinanto, A. et al, “End-to-end available bandwidth probing inheterogeneous IP home networks”, Consumer Communications and NetworkingConference (CCNC), 2011 IEEE, pp. 431-435, 9-12 Jan. 2011. This papershows that PGM can be used to determine bottleneck link speeds inheterogeneous networks consisting of links differing in speeds andmedium, for example wired links and wireless links etc.

While PGM-probing provides the capacity and available bandwidth of alayer-2 bottleneck link in a layer-3 network path, in a multi-linenetwork route it cannot ascertain which link is the bottleneck link.

What is needed in the art is to find out as much as possible about thenetwork being probed.

SUMMARY OF THE INVENTION

For finding out more about the network, according to one aspect of anembodiment of the present invention, a method is disclosed of probing anetwork route using a pair of probe packets comprising a first probepacket I1 with size L1 and a second probe packet I2 with size L2, and inwhich the size of I1 is less than or equal to the size of I2, in otherwords L1≦L2.

The method comprises transmitting the first pair of probe packets I1 andI2 back-to-back onto the network route, in other words the second packetI2 is transmitted as soon as is possible, or without delay, after thetransmission of the first probe packet I1 in any pair of probe packets.In practice this may occur if the transmission of the second packetsoccurs within a clock cycle of the first packet clearing the first linkin the network route. This means that the first probe packet I1 istransmitted before the second probe packet I2. Because each probe packetin the pair of probe packets has a size a ratio of their sizes can bedefined as [L1/L2]. The method further includes transmitting onto thenetwork route at least one further pair of probe packets back-to-back,m1 with size M1 and m2 with size M2, in which the size of m1 is lessthan or equal to the size of m2, in other words M1≦M2, and further wherea ratio [M1/M2] can be defined and is different from, in other wordsnumerically different from, the ratio [L1/L2]. The method furtherincludes deriving an indicator of the time taken for the second probepacket in the probe packet pairs to traverse the network route, andidentifying if the values of the derived indicators vary or aresubstantially the same.

If the values of the derived indicators are substantially the same thenthe first link in the network route is the bottleneck link and animportant piece of information has been derived about the network.

Embodiments of the present invention are particular advantageous in theprobing of a home, or domestic, network because a common networktopology includes a link from a network gateway device to a router withseveral devices, for example home computers, laptops and other domesticdevices positioned behind the router. The bottleneck link in the networkmight be the link between the gateway device and the router but probingto and from devices and between devices is unlikely to indicate this.With knowledge that this is the bottleneck link, however, it is possibleto make network management decisions, for example allowing swiftertransfer of data and information by transmitting it between devices onthe network behind the gateway rather than transmitting it from thegateway to each device in turn. For example, if a gateway device canascertain that the link between itself and the router is the bottlenecklink a decision can be taken to transfer data to be distributed to asingle device behind the router and then to distribute it out to otherdevices from that one device, instead of attempting to transmit itmultiple times from the gateway to each recipient device and sufferingthe network bottleneck link each time.

Conversely, if the values of the derived indicators vary then it can beassumed that the first link in the network route is not the bottlenecklink and again, an important piece of information has been learned.

There are several ways of deriving an indicator of the time taken forthe second probe packets in the probe packet pairs to traverse thenetwork route.

In one embodiment multiple pairs of probe packets are transmittedback-to-back over the network route and the transit times for the secondpacket of each pair recorded. The minimum value of transit time for thesecond packet is used to derive the indicator of time taken for thesecond probe packet. In other words for any transmitted pair of probepackets with first probe packet n1 with size N1, and second probe packetn2 with size N2, the method further includes repeating measurements ofpairs of probe packets n1 and n2, and deriving the indicator based onthe minimum value of time taken for the second probe packet n2 totraverse the network route over the repeated pairs of probe packets n1and n2.

As an alternative, the second probe packets in any two pairs of probepackets can be arranged to be the same size as each other. In this casethe first packets of each pair would differ in size from each other inorder to comply with the criterion that the ratio [L1/L2] is not equalto the ratio [M1/M2]. When both second packets in the two probe pairsare the same size as each other the time taken for each second packet totravel the route under test can be expected to be the same and the timetaken for the second probe packets to traverse the route under test canbe used as the derived indicator. In other words for probe packets I2and m2, and when the condition L2=M2 is satisfied, the derived indicatoris the time taken for the second probe packet to traverse the networkroute.

In a further embodiment the derived indicator is derived from the sizeof the second packet and the time taken for the second packet totraverse the route. Further, the derived indicator can be calculatedfrom the ratio of the size of the second packet to the time taken to forthe second packet to traverse the route.

The condition that the size of packet L1 is less than or equal to thesize of packet L2, in other words that size L1≦ size L2, is typicallysatisfied if the length of packet L1, as would be understood by theskilled person, is less than the length of packet L2, or in other wordslength L1≦ length L2.

Typically the probe pairs are transmitted as layer-2 traffic.

As will be understood by the skilled person the packet pairs travel overa route in a network. The network comprises network paths, or routes,comprising one or more network links defined between two devices,sometimes referred to as end devices, or alternatively can be a routedefined from a device A over a series of links in the network to adevice B and can also be defined from device A to device B and then backover the same links to A. The latter case is often called a round-triproute. As is known in the art there are typically two ways in which aprobe packet can be sent round-trip, either a packet is sent out to adestination device and the same packet is sent back from the destinationto the origin, or the probe packet is sent out and the destination sendsa reply packet back to the origin immediately after having received theprobe packet, where the reply packet is different from the originalprobe packet, but is of known size. The invention can be used on eitherround-trip or one way routes. When the network route to be probed is around-trip route, and in this case each pair of probe packets is sentround-trip on the network, then the time taken to traverse the networkroute is a round-trip time.

In an embodiment, in order to maximise the different size ratiospossible for the probe packets it is particularly useful if the size ofthe second probe packet in any pair of probe packets is the maximum sizesupported by the network route.

According to another aspect of the present invention, a gateway devicewhich works the method as described is provided. A gateway devicecouples a first network to a second network, and is typically used tocouple a home or domestic network to the wider internet, the gatewaydevice acting as a bridge between the home network and the internetoutside the home network. In this case the first network is the homenetwork. The gateway device is configured to probe a network route inthe first network using a probe packet pair. In this case the probe paircomprises a first probe packet I1 with size L1 and a second probe packetI2 with size L2, and wherein the size of the first probe packet I1 issmaller than the size of the second probe packet I2. In other words sizeL1≦ size L2. The gateway is further configured to transmit the firstpair of probe packets I1 and I2 back-to-back, in other words the secondpacket I2 is transmitted as soon as is possible, or without delay, afterthe transmission of the first probe packet I1 in any pair of probepackets, onto the network route. The gateway is additionally configuredto transmit onto the network route at least one further pair of probepackets back-to-back, m1 and m2. Probe packet m1 has size M1 and probepacket m2 has size M2 and the size of the m1 is less than or equal tothe size of m2, in other words M1≦M2. Ratios can be defined for probepackets I1, I2, m1, and m2 and the gateway is configured to transmitprobe packets I1, I2, m1 and m2 wherein the ratio defined by [M1/M2] isdifferent from the ratio defined by [L1/L2].

Further, the gateway is configured to derive an indicator of the timetaken for the second probe packet in the probe packet pairs to traversethe network route, and further identify if the values of the derivedindicators vary or are substantially the same.

If the values of the derived indicators are substantially the same thenthe first link in the network route in the first network is thebottleneck link and an important piece of information has been derivedabout the first network. If the values of the derived indicators varythen it can be assumed that the first link in the network route is notthe bottleneck link and again, an important piece of information is nowknown.

Embodiments of the present invention are particular advantageous if thefirst network is a home, or domestic, network because these networks aretypically remote from service providers providing data streams into thehome and yet the topology and arrangement of links within these domesticnetworks might critically affect the flow of data streams within them.Knowledge of the capacity and arrangement of links in domestic networksallows valuable data management decisions to be taken and worked whenorganising data flow behind the gateway in the domestic network. Themethod as described can be worked on and from the gateway eitherautomatically, or managed from outside the first network in which theprobed route is situated. Typically, the method can be performed bysoftware which can be installed on the gateway and this can be installedeither in-situ by a locally performed software installation or upgrade,or can alternatively by downloaded onto the gateway from the widerinternet.

While, in the case of domestic networks, it is particularly advantageousto perform the method from a suitably configured gateway device coupledto the network, there are other possibilities. For example, in anotheraspect of the present invention, the method can also be performed fromany other device in the domestic network, for example a computerconnected in a domestic network or any hand held computing device oreven a mobile device temporarily connected into the network. In thiscase suitable software should be installed on the computer, or otherdevice, to allow both performance of the method but also allow resultsto be transmitted to an entity outside the home network, for example toa service provider, if that is required.

In this case the method is performed from a device configured to probe anetwork route in a network, the device configured to probe the networkroute using a probe packet pair, the probe pair comprising a first probepacket I1 with size L1 and a second probe packet I2 with size L2, and inwhich the size of L1 is less than or equal to the size of L2. The deviceis also configured to transmit the first pair of probe packets I1 and I2back-to-back, in other words the second packet I2 is transmitted as soonas is possible, or without delay, after the transmission of the firstprobe packet I1 in any pair of probe packets, onto the network route,the pair of probe packets having a ratio [L1/L2], and further configuredto transmit onto the network route at least one further pair of probepackets back-to-back, m1 and m2, wherein the size of M1 is less than orequal to M2 but where further the ratio [M1/M2] is different from theratio [L1/L2]. The device is also configured to derive an indicator ofthe time taken for the second probe packet in the probe packet pairs totraverse the network route, and further identify if the values of thederived indicators vary or are substantially the same.

As an alternative the method can be performed from outside the domesticnetwork entirely, from a device, such as a server or other computer or amobile device, remote even from the gateway coupling the domesticnetwork to a wider network such as the internet.

Therefore, according to yet another aspect of the present invention atest apparatus configured to probe a network route in a remote networkaccessible through a gateway comprised in the remote network isdisclosed. The test apparatus is configured to work the described methodand may be, for example, a server, a computer, a linked network ofcomputing devices, a laptop or other device with software capability.

In this case the test apparatus is configured to probe the network routevia the gateway using a probe packet pair in which the probe paircomprises a first probe packet I1 with size L1 and a second probe packetI2 with size L2. The first probe packet I1 has a size which is smallerthan the second probe packet I2, in other words the criterion L1≦L2 issatisfied. The test apparatus is further configured to transmit thefirst pair of probe packets I1 and I2 back-to-back, in other words thesecond packet I2 is transmitted as soon as is possible, or withoutdelay, after the transmission of the first probe packet I1 in any pairof probe packets, onto the network route. The test apparatus is furtherconfigured to transmit onto the network route at least one further pairof probe packets back-to-back, m1 and m2, with respective sizes M1 andM2 and where the size M1 of m1 is less than or equal to the size M2 ofm2. In other words the criterion M1≦M2 is satisfied. Again, ratios[L1/L2] and [M1/M2] may be defined for probe packets I1, I2, m1 and m2and the ratio [M1/M2] is different from the ratio [L1/L2] fortransmitted probe packets. The test apparatus is further configured toderive an indicator of the time taken for the second probe packet in theprobe packet pairs to traverse the network route, and further configuredto identify if the values of the derived indicators vary or aresubstantially the same.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments are described in the Figures.

FIG. 1 gives an example of a typical network path on which the inventioncan be used.

FIG. 2 is a diagram showing transmission of two packets over a simplenetwork route, the smaller packet being transmitted first.

FIG. 3A shows a scenario demonstrating use of the described method.

FIG. 3B shows a scenario demonstrating use of the described method.

FIG. 4A shows results of use of the described method.

FIG. 4B shows results of use of the described method.

FIG. 5 shows typical data values associated with a use of the method.

FIG. 6 shows an embodiment for working the invention.

DETAILED DESCRIPTION

FIG. 1 gives an example of a network path suitable for analysis andprobing by the method described. The network path, or route, in thisinstance contains a linked series of devices including a laptop 101, amodem 102, two switches 103 and 106, two routers 104 and 105, and aserver 107. All these devices are linked together in a manner as isknown in the art. Switches 103 and 106 are layer 2 devices, for exampleEthernet devices and switch 103 can for example be a DSLAM. Routers 104and 105 are layer 3 devices, for example IP routers. In this networkpath the capacity of the link between the laptop 101 and modem 102 is100 Mbit/s. The capacity of the links between the switch 103 and router104 and between the switch 106 and server 107 are also each 100 Mbit/s.The capacity of the links between routers 104 and 105 and between router105 and switch 106 are both 1 Gbit/s.

However the capacity of the link between the modem 102 and the switch103 is only 10 Mbit/s and is the bottle neck link on the entire route.In other words the link between the modem 102 and the switch 103 has thesmallest, or lowest, capacity of all the links in the defined, ordescribed, route.

A PGM probe of the prior art, as is known by the skilled person, candiscover that the capacity or available bandwidth of the network pathbottleneck is only 10 Mbit/s, but a PGM probe cannot identify which ofthe network links in the network route is the bottleneck link thatyields this 10 Mbit/s capacity.

Other probing techniques, such as VPS probing, are able to probe on alink-by-link basis, but only on layer-3 links, for example IP linksbetween 2 routers. In this example a VPS probe would only be able toprobe between router 104 and router 105 and cannot distinguish betweenthe link that connects modem 102 with switch 103 and the link thatconnects switch 103 with router 104. Neither can it distinguish betweenthe link that connects router 105 with switch 106 and the link thatconnects switch 106 and server 107. In effect a VPS prove would returnresults that give an overall capacity for combinations of links, in thiscase for the combination of the links between modem 102 and router 104and further for the combination of the links between router 105 andserver 107. VPS will treat these combinations of links as single linksand therefore yield a result which is representative for none of theindividual links which are comprised within them. The measured result,as identified by a VPS probe, for a combination of links will be lowerthan the actual lowest speed of the combined links because measurementsare based on delay and combining links means adding up the delays ofthose links.

Therefore using only methods known in the art we see that even if probesare sent from the modem 102 in the network route and the bottleneck linkis the first link between the modem 102 and ultimately the server 107 towhich a probe is sent, it is impossible to place the bottleneck linkusing known previously methods. In other words known methods do notallow the discovery that the bottleneck link on the path from the modem102 to the server 107 is actually this first link in that path.

The method here described allows determination of whether the first linkon a network path is the bottleneck link of that path, even if thatfirst link is a layer 2 link.

A solution to this problem involves sending a probe of two packets,back-to-back on the network, in other words with the second packet sentimmediately after the first, the two packets having different sizes,typically defined in bytes, although other definitions of size may beused, and in which the smaller packet is sent first.

Typically while the probe is layer-3 traffic the probe packets arelayer-2 traffic.

Because the smaller packet is sent first this first packet is generallyexpected to traverse the various links faster than the second packet.This will mean that the two packets will spread out, in other wordsdisperse, with the larger and therefore slower second packet nevercatching up with the faster first packet on any of the links in thenetwork route over which the packets are sent. This can be used todetermine if the first link in the network path is the bottleneck linkor not. This assumes a situation in which there is no cross traffic, inother words no other traffic running on the network.

FIG. 2 shows what happens when two packets are transmitted over a simplenetwork route, the smaller packet being transmitted first. Two packets,PK1 201 and PK2 202 are transmitted from source 203 over a network routedefined by network device 204 and receiver 205. In this example the twoprobe packets are sent over a round trip route because the source 203,or the prober, sends the first packet PK1, 201, and the second packetPK2, 202, to receiver 205 via device 204 and then the receiver 205replies. As will be understood by the skilled person, the vertical linesrepresent time developing from a nominal value t₀ at the top to the endof the probe measurement at the bottom of FIG. 2 in which all repliesare received back at source 203 at some time t_(n) later. The sameoverall procedure is also valid for one-way probing in which the prober,in this case source 203, or prober, sends probe packets to the receiver,or destination node, 205, without replies going back to the prober 203.In this latter case the packets 201 and 202 will arrive at thedestination receiver 205 at some intermediate time t₁ and t₂. Generallythe time at which a packet is deemed to have arrived, or been received,is the time at which the functional information in the packets has beenwholly received and can be put to purpose.

It can be seen from FIG. 2 that how the packets behave in relation toeach other in fact depends on the location of the bottleneck link andthe speeds of the various different links in the probe path. Thisdependence is what is used in this invention to determine if the firstlink is the bottleneck link.

The key network effect that our invention uses is the following. If thefirst link is the bottleneck link, then the second packet will not bedispersed by the first packet on any of the network links, irrespectiveof the size of the first packet. That is, only the departure time of thesecond packet will depend on when the first packet has left the senderand the arrival time will change accordingly. This means that thederived indicator will have the same value irrespective of the ratio ofthe two packet sizes in the probe pair.

If instead the bottleneck link is further down the probed path, thesecond packet might suffer additional delay from the first packet at thebottleneck link. In other words the packets will be dispersed. This willdepend on the ratio between the sizes of the two probe packets and thespeeds of the various network links. If in a probe method two equalpacket sizes were used then the two probe packets will be dispersed atthe bottleneck link, however, if the first packet is much smaller thanthe second packet, it may have traversed the bottleneck link alreadywhen the second packet arrives at that link. In that case, no dispersiontakes place. So, for very large ratios, which occurs when the twopackets have almost the same size, the packet will always get dispersedat the bottleneck link, while for very small ratios, which occurs whenthe first packet is much smaller than the second packet, they will not,even if passing through the bottleneck link.

Dispersion can also happen at non-bottleneck links but this has noimpact on the method and dispersion does not necessarily influence theresult of the derived indicator.

In practice an indicator can be calculated and uses the ratio betweenthe second packet size and the minimum round-trip-time for that packet,as an indicator for decision making regarding the question if the firstlink is the bottleneck or not.

${Indicator} = {\frac{L\; 2}{{RTT}\; \min} = \frac{L\; 2}{\min\limits_{\lbrack{i = {1.\mspace{14mu}.n}}\rbrack}\left\lbrack {{Ta}_{{PK}\; 2{(i)}} - {Td}_{{PK}\; 2{(i)}}} \right\rbrack}}$

where L2 is the length of the second packet, RTTmin is the minimumround-trip time for the second packet, Ta is its arrival time and Td isits departure time.

If the first link is not the bottleneck link, the observed value for theindicator will not be the same for all packet size ratios. For largeratios L1/L2, the second packet will be dispersed somewhere on the probepath, its arrival time will increase, and the indicator will becomesmaller.

This allows us to determine if the first link is the bottleneck linkbecause if the indicator remains the same for all packet pair sizeratios, then the first link is the bottleneck link.

In practice two packets, PK1, a smaller packet 201, is transmitted ontothe network route before PK2, a larger packet 202, and the network pathis probed by this packet-pair. Then, after having observed theround-trip times probing is repeated a number of times whilst the ratiobetween the packet sizes is varied. Varying the ratio can be performedby keeping the second, and larger, packet at the maximum size possiblefor the network route. In practice this can be achieved by maintainingthe size of the second packets at the maximum transmission unit (MTU) ofthe probe path and the skilled person will know how to achieve this. Thesize of the first packet is then varied from small to large or viceversa. These steps are repeated a number of times to remove the effectsof cross traffic.

Therefore as is described above and as is shown in FIG. 2, if thebottleneck link is the first link in the network route then the arrivaltime will change accordingly, but the denominator in the equation willnot change. This means that the indicator will have the same valueirrespective of the ratio of the two packet sizes in the probe pair.Further, L2 remains the same throughout all measurements, the indicatorcan just be RTTmin.

As an example, the method allows us to differentiate between the twofollowing scenarios which are quite similar.

FIGS. 3A and 3B show a short network route between a computing device301 and a server 304. The route is made up of a series of links betweenthe device 301 and the switch 302, the switch 302 and the switch 303,and finally the switch 303 and the server 304.

In FIG. 3A the link between device 301 and the switch 302 has a capacityof 40 Mbit/s; the link between the switch 302 and the switch 303 has acapacity of 100 Mbit/s; and the link between the switch 303 and theserver 304 has a capacity of 50 Mbit/s.

In FIG. 3B the link between device 301 and the switch 302 has a capacityof 100 Mbit/s; the link between the switch 302 and the switch 303 has acapacity of 40 Mbit/s; and the link between the switch 303 and theserver 304 has a capacity of 50 Mbit/s.

In other words the link speeds of the first and second links in theroute are swapped in the two examples.

Using VPS probing would reveal the same value, incidentally ofapproximately 18 Mbit/s, for both routes. The same is true for PGMprobing, which would return a result of 40 Mbit/s for the measurement ofcapacity. Additionally, neither probing method would allow us to findthe location of the bottleneck link.

Use of the method as described to probe both routes using a secondpacket size described by L2=10000 bits, and varying the size of thefirst packet L1 between 1000 bits and 9500 bits, in increments of 500bits. Because L2 has a fixed value, we can just use the Round-Trip-TimeRTT of the second packet as the indicator. In this example we useround-trip-probing, as opposed to probing from point A to point B. Theresults are shown in FIGS. 4A and 4B.

FIG. 4A shows the RTT of the second packet as a function of L1, where L1is the size of the first packet, for the scenario described in FIG. 3A.

FIG. 4B shows the RTT of the second packet as a function of L1, where L1is the size of the first packet, for the scenario described in FIG. 3B.

For the scenario of FIG. 3A we see that the RTT is constant, it does notdepend on L1. This is because the bottleneck link is first, so thesecond packet is never delayed by the first one on the probe path, afterit is sent out by the probe node, in this case computing device 301.

For the scenario of FIG. 3B we see that for sizes of the first packet upto L1=4000 bits, the graph is the same as the one for scenario A. Afterthat, the first packet becomes so large that it has not passed thebottleneck link entirely before the second packet arrives there. Thus,the second packet becomes delayed a bit by the first packet, causing alonger RTT. The bigger the first packet gets, the longer it will delaythe second packet and this is seen in the rising graph. Additionally,the levelling in the graph happens at the turning point at which theratio of packet size equals the ratio of the capacity of the bottlenecklink to the capacity of the first link, described by the equation

L1/L2=capacity of the bottleneck link/capacity of the first link.

The capacity of the bottleneck link can be retrieved by using PGMprobing, and from this one would be able to calculate the capacity ofthe first link.

For the scenario of FIG. 3B and in the case in which packets sizes aredefined by L1=4500 bits and L2=10000 bits, FIG. 5 shows the departuretime from the nodes at one side of the link and the arrival time at thenodes on the other side of the link. A probe packet can depart on a linkif it has arrived at the node from which it can enter the link and ifadditionally the link is free. In other words a probe packet can begintransmission across a link if the previous probe packet has cleared thatlink already. For the second probe packet this means the first probepackets must have crossed the link entirely.

For the first probe packet, any link in the route will nominally alwaysbe free, because there are no prior packets from the method on the link.This assumes that cross traffic does not interfere. In practice crosstraffic does interfere with measurements but the effect can be mitigatedby repeating the measurement a number of times and taking the minimummeasurement.

In the table of FIG. 5, which shows typical values for probe packets P1and P2 as they progress across a network route comprising 3 links A, Band C, we can see that when the second probe packet P2 arrives at linkB, at t=0.0145, the first probe packet P1 has not crossed the linkentirely, so P2 has to wait until the first probe packet P1 has clearedthe link entirely, at t=0.01575.

When performing the method in practice it is not always easy to knowbeforehand the correct minimum L1 to choose. A way to proceed is tostart with the first and second packet sizes roughly equal, in otherwords satisfying L1≈L2, and then start decreasing the size L1incrementally, or little by little. At first, there may not be muchdifference in the minimum value of the round-trip time for the secondpacket, RTT2, but as L1 gets smaller, the minimum value of RTT2, ormin(RTT2), should get smaller in a linear relationship with L1, as shownin FIG. 4B. If this does not occur, if min(RTT2) stays the same for allvalues of L1, then it can be concluded that the first link is thebottleneck link.

If anomalies in results occur during this reduction in size of L1, thenmost likely L1 has been decreased too much. Such anomalies can occur,for example, due to other network effects.

FIG. 6 shows a flow chart of a useful embodiment for working theinvention in practice. At start 601 a counter i is set at zero 602 and aprobe is performed 603 with first packet size L_(i,1) smaller or equalto the second packet size L_(i,2). Regardless of the actual size L_(i,1)and L_(i,2) the probe pairs are repeated k times in order to identifythe minimum round-trip times (RTTs) for this combination of packet size.After the repetition the counter i is moved on to i=1 at 604.

Following this, 605, a new pair of probes is produced in which the firstpacket size L_(i,1) smaller or equal to the second packet size L_(i,2)but with the additional requirement that the ratio of the sizes of thenew first to second packet is not equal to the ratio of the sizes of theprevious first to second packet. In other words:

L_(i,1)/L_(i,2)≠L_(i-x-1)/L_(i-x-2)

for which x=1, . . . , i.

Step 605 is also repeated k time.

In step 606 an indicator, A_(i) is calculated for all pairsL_(i,1)/L_(i,2), which also includes pairs L_(i-x-1)/L_(i,x-2).Alternatively indicators can be calculated as pairs are transmitted andresults become available, in other words indicators for original probepairs L_(i,1) and L_(i,2) sent during 603 may be calculated beforesubsequent probe pairs are transmitted during 605.

In step 607 it is determined if the indicator for the first probe pairs,sent during 603, is equal to the indicator for subsequent pairs ofprobes, sent during 605. In other words it is determined if:

A_(i)≠A_(i-x)

for which x=1, . . . , i.

If the answer to this determination is yes, then the first link is notthe bottleneck link, 608. If the answer to this determination is no,then it must be considered 609 if

i=i_(max)

If no then the process is returned to step 604, the counter is increasedby 1, another pair of probes is produced with size ratio not equal toany previous pair of probes and transmission of the probe pair isrepeated k times to identify the minimum RTT. However if yes, then theprocess moves on to step 610 and it is ascertained if in the resultsgenerated there are indicators A_(i) and A_(j) for probe pairsL_(i,1)/L_(i,2) and L_(j,1)/L_(j,2) where:

L_(i,1)/L_(i,2)<<L_(j,1)/L_(j,2)

If the answer is yes then the first link is the bottleneck, 611. If theanswer is no then there is no result, 612. In the latter case the methodof this embodiment can be reworked with a greater number of repeatedprobe pairs k, or with a larger value of i.

1. A method of probing a network route using a pair of probe packetscomprising a first probe packet I1 with size L1 and a second probepacket I2 with size L2, and wherein L1 is smaller than or equal to L2,the method comprising: transmitting the first pair of probe packets I1and I2 back-to-back onto the network route, the pair of probe packetshaving a ratio [L1/L2]; transmitting onto the network route at least onefurther pair of probe packets back-to-back, m1 with size M1 and m2 withsize M2, wherein M1 is smaller than or equal to M2 and further whereinthe ratio [M1/M2] is different from the ratio [L1/L2]; deriving anindicator of the time taken for the second probe packet in the probepacket pairs to traverse the network route; and identifying whether thevalues of the derived indicators are either different or substantiallythe same.
 2. The method of claim 1, further comprising: for any giventransmitted pair of probe packets having a first probe packet n1 withsize N1, and a second probe packet n2 with size N2, repeatingmeasurements of pairs of probe packets n1 and n2, deriving the indicatorbased on a minimum value of time taken for the second probe packet n2 totraverse the network route over the repeated pairs of probe packets n1and n2.
 3. The method of claim 1 wherein for probe packets I2 and m2, acondition L2 is equal to M2 is satisfied, and further wherein thederived indicator is the time taken for the second probe packet totraverse the network route.
 4. The method of claim 1 wherein the derivedindicator is derived from the size of the second packet and the timetaken for the second packet to traverse the route.
 5. The methodaccording to claim 4 wherein the derived indicator is calculated fromthe ratio of the size of the second packet to the time taken to for thesecond packet to traverse the route.
 6. The method of claim 1, whereinL1 being smaller than or equal to L2 corresponds to the length of L1being smaller than or equal to the length of L2.
 7. The method of claim1 wherein transmitting any given pair of probe packets back-to-back intothe network, including the first pair and the at least one further pair,comprises transmitting a first probe of the any given pair into thenetwork followed, without any delay, by transmitting a second probe ofthe any given pair into the network.
 8. The method of claim 1, whereinthe probe pairs are transmitted as layer-2 traffic.
 9. The method ofclaim 1, wherein the network route is a round-trip route and furtherwherein each pair of probe packets is sent round-trip on the network andwherein the time taken to traverse the network route is a round-triptime.
 10. The method of any of claim 1, wherein each pair of probepackets is transmitted onto the network route one-way.
 11. The method ofclaim 1, wherein the size of the second probe packet in any pair ofprobe packets is the maximum size supported by the network route.
 12. Agateway device for a first network, wherein the gateway device isconfigurable for coupling the first network to a second network andwherein the gateway is configured to probe a network route in the firstnetwork using a probe packet pair, the probe pair comprising a firstprobe packet I1 with size L1 and a second probe packet I2 with size L2,and wherein L1 is smaller than or equal to L2, and wherein the gatewayis configured to: transmit the first pair of probe packets I1 and I2back-to-back onto the network route, the pair of probe packets having aratio [L1/L2]; transmit onto the network route at least one further pairof probe packets back-to-back, m1 and m2, and wherein m1 has size M1,wherein m2 has size M2 and further wherein M1 is smaller than or equalto M2 and further wherein the ratio [M1/M2] is different from the ratio[L1/L2 ]; derive an indicator of the time taken for the second probepacket in the probe packet pairs to traverse the network route; andidentify whether the values of the derived indicators are eitherdifferent or substantially the same.
 13. A test apparatus configured toprobe a network route in a remote network which is accessible through agateway comprised in the remote network, the test apparatus beingconfigured to probe the network route via the gateway using a probepacket pair, the probe pair comprising a first probe packet I1 with sizeL1 and a second probe packet I2 with size L2, and wherein L1 is smallerthan or equal to L2, and wherein the test apparatus is configured to:transmit the first pair of probe packets I1 and I2 back-to-back onto thenetwork route, the pair of probe packets having a ratio [L1/L2];transmit onto the network route at least one further pair of probepackets back-to-back, m1 and m2, wherein M1 is smaller than or equal toM2 and further wherein the ratio [M1/M2] is different from the ratio[L1/L2]; derive an indicator of the time taken for the second probepacket in the probe packet pairs to traverse the network route; andidentify whether the values of the derived indicators are eitherdifferent or are substantially the same.
 14. A device configured toprobe a network route in a network, the device configured to probe thenetwork route using a probe packet pair, the probe pair comprising afirst probe packet I1 with size L1 and a second probe packet I2 withsize L2, and where L1 is smaller than or equal to L2, and wherein thedevice is configured to: transmit the first pair of probe packets I1 andI2 back-to-back onto the network route, the pair of probe packets havinga ratio [L1/L2]; transmit onto the network route at least one furtherpair of probe packets back-to-back, m1 and m2, wherein M1 is smallerthan or equal to M2 and further wherein the ratio [M1/M2] is differentfrom the ratio [L1/L2]; derive an indicator of the time taken for thesecond probe packet in the probe packet pairs to traverse the networkroute; and identify whether the values of the derived indicators areeither different or substantially the same.
 15. non-transientcomputer-readable medium having instructions stored thereon that, whenexecuted by one or more processors, cause the one or more processors tocarry out operations for probing a network route using a pair of probepackets comprising a first probe packet I1 with size L1 and a secondprobe packet I2 with size L2, wherein L1 is smaller than or equal to L2,and wherein the operations include: transmitting the first pair of probepackets I1 and I2 back-to-back onto the network route, the pair of probepackets having a ratio [L1/L2]; transmitting onto the network route atleast one further pair of probe packets back-to-back, m1 with size M1and m2 with size M2, wherein M1 is smaller than or equal to M2 andfurther wherein the ratio [M1/M2] is different from the ratio[L1/L2];deriving an indicator of the time taken for the second probe packet inthe probe packet pairs to traverse the network route; and identifyingwhether the values of the derived indicators are either different orsubstantially the same.